The present invention relates generally to systems for dispensing cryogenic gases from vessels storing cryogenic liquids and, more particularly, to a dispensing system for cryogenic liquid vessels that provides cryogenic gas at high pressures and high flow rates.
Cryogenic gases are used in a variety of industrial and medical applications. Many of these applications require that the cryogen be supplied as a high pressure gas. For example, high pressure nitrogen and argon gases are required for laser welding while high pressure nitrogen, oxygen and argon gases are required for laser cutting. Such cryogens are typically stored as liquids in vessels, however, because one volume of liquid produces many volumes of gas (600-900 volumes of gas per one volume of liquid) when the liquid is permitted to vaporize/boil and warm to ambient temperature. To store an equivalent amount of gas requires that the gas be stored at very high pressure. This would require heavier and larger tanks and expensive pumps or compressors.
Cryogenic vessels typically consist of an insulated double-walled tank for storing cryogenic liquids. It is impossible, however, to prevent all heat transfer between the interior of the tank and the external environment. As a result, the cryogenic liquid in the tank will slowly expand, and eventually vaporize, so as to pressurize the tank. Cryogenic gas will collect in the head space of the tank. Because the cryogen is used as a gas, it is advantageous to use the gas in the head space before vaporizing the liquid within the tank. Using gas from the head space reduces pressure in the head space so that venting may be avoided.
A disadvantage with supplying cryogenic gas from the head space of the tank is that the head space pressure will not always be sufficient to meet the use requirements. When the head space pressure is insufficient, the liquid in the tank must be vaporized to meet the use requirements and rebuild the head space pressure.
A prior art dispensing system that coordinates the supply of cryogenic gas from the head space and liquid body of a tank is illustrated in FIG. 1. Such a system is available from Chart Inc. of Burnsville, Minn., owner of the present application. As illustrated in FIG. 1, the system includes a cryogenic liquid storage vessel, indicated in general at 6, including an inner tank 8 and a jacket 10. The inner tank 8 holds a supply of cryogenic liquid, shown at 12. The head space 14 of the inner tank contains cryogenic gas that forms due to the transfer of heat between the interior of the inner tank of the vessel and the external environment.
To further pressurize the vessel, a pressure builder coil 16 is connected to the bottom of inner tank 8 via liquid feed and trap 17 and helically disposed around inner tank 8 and in contact with jacket 10. The cryogenic liquid is free to flow from the inner tank into coil 16. Because pressure building coil 16 is in contact with the jacket 10 of the vessel, heat transfer between the external environment and the liquid in coil 16 will be relatively great. As a result, the cryogenic liquid in the coil will be vaporized. Coil 16 is connected to pressure building line 18 which communicates with the head space 14 of vessel 6 via a circuit 19 that includes a pressure building valve 20 and a pressure building regulator 22 (preferably a diaphragm regulator) and a return line 24.
In addition, return line 24 connects the head space 14 of vessel 6 to a by-pass line 26 containing an economizer regulator 28. By-pass line 26 and regulator 28 connects circuit 19 and return line 24 to a vaporizer line 29 and a vaporizer 30 that is connected to a gas use line 32. Like coil 16, vaporizer 30 is helically disposed about the inner tank 8 and connected to the jacket 10 of vessel 6 such that heat transfer to vaporizer 30 will be great enough to vaporize any cryogenic liquid therein before it is delivered to gas use line 32. Gas use line 32 includes a gas use valve 34 for controlling the delivery of gas for its intended use. A dip tube 36 having a lower end positioned within the cryogenic liquid 12 also communicates with vaporizer line 29, and thus, vaporizer 30 and gas use line 32.
Economizer regulator 28 is set to close when the pressure within the head space 14 of the vessel 6 drops below a first predetermined level. Pressure building regulator 22 is set to open when the pressure within the head space drops below a second predetermined level that is lower than the first predetermined level at which the economizer regulator 28 is set to close. As such, pressure building and economizer regulators 22 and 28, respectively, are never open simultaneously.
To dispense cryogenic gas, gas use valve 34 is opened. When the pressure within the head space 14 of vessel 6 is above the pressure setting of economizer 28, so that economizer 28 is open, gas travels from the head space 14 of the vessel 6 through return line 24, by-pass line 26, vaporizer line 29, vaporizer 30 and ultimately to gas use line 32.
If the pressure of head space 14 falls below the first predetermined value set at economizer regulator 28, economizer 28 will close. Under such circumstances, when gas use valve 34 is open, cryogenic liquid is withdrawn from the inner tank 8 via dip tube 36 and directed to vaporizer coil 30. The liquid is converted to gaseous cryogen in the vaporizer coil and is delivered to valve 34 via use line 32.
If the pressure of head space 14 falls below the second predetermined value set at pressure building regulator 22, regulator 22 opens and, if pressure building valve 20 is open, liquid is removed from the bottom of inner tank 8 via feed 17 and enters pressure building coil 16 where it is vaporized and delivered to pressure building line 18. The resulting gas will flow through circuit 19 and, because economizer regulator 28 is closed, through return line 24 so that the head space 14 is pressurized.
Industrial applications such as laser welding and cutting require that the cryogenic gases be provided simultaneously at high pressures and flow rates. Advances in industrial laser technologies have resulting in demands for increased flow rates. Pressures in the range of approximately 400-420 psig and flow rates in the range of approximately 1500-2500 scfh are now typical. While the system described above is effective at dispensing gases at such pressure levels, and indeed up to around 500 psig, it encounters difficulties in maintaining these operating pressures at such high flow rates.
Accordingly, it is an object of the present invention to provide a gas dispensing system for cryogenic liquid vessels that is capable of delivering high pressure gas at high flow rates.
It is another object of the present invention is to provide a gas dispensing system for cryogenic liquid vessels that may be retrofitted to earlier cryogenic liquid vessels and gas dispensing systems.
It is another object of the present invention to provide a gas dispensing system for cryogenic liquid vessels that is economical to operate.
It is still another object of the present invention to provide a gas dispensing system for cryogenic liquid vessels that is inexpensive to produce and maintain.
Other objects and advantages will be apparent from the remaining portion of this specification.
The present invention is directed to a system for dispensing pressurized cryogenic gas at high flow rates. The system includes a storage vessel having a jacket surrounding an inner tank that contains a supply of cryogenic liquid with a head space there above. An internal pressure builder coil is positioned between the jacket and inner tank, is in contact with the jacket and is helically positioned about the inner tank. An external pressure building heat exchanger is in communication with the internal pressure builder and the head space of the inner tank of the storage vessel. As a result, cryogenic liquid from the inner tank flows into the internal pressure builder coil and is, as a result of heat added by the internal pressure builder, at least partially vaporized so that a gas and liquid mixture is produced. This produces a pumping action so that the gas and liquid mixture is driven to the external pressure building heat exchanger where the liquid is vaporized and the gas is heated. The resulting heated gas is delivered to the head space of the inner tank so that the inner tank is pressurized. A pressure building regulator is in circuit with the external pressure building heat exchanger and the internal pressure builder coil and opens to allow liquid to enter the internal pressure builder coil when the pressure within the head space of the vessel drops to a predetermined level.
A dip tube is in communication with the cryogenic liquid within the inner tank and a vaporizer is in circuit between a use line and the dip tube. The vaporizer coil may be positioned between the inner tank and the jacket, and in contact with the latter, or external to the tank. Liquid from the inner tank flows through the dip tube and the vaporizer so that gas produced thereby may be dispensed from the use line. The vaporizer selectively communicates with the dip tube and is selectively in communication with the head space of the vessel through an economizer regulator so that when the economizer regulator is open, gas from the head space flows through the vaporizer to the use line and when the economizer regulator is closed, liquid from the inner tank travels through the dip tube and vaporizer so that gas is produced and provided to the use line.
The following detailed description of embodiments of the invention, taken in conjunction with the appended claims and accompanying drawings, provide a more complete understanding of the nature and scope of the invention.